First photoresponsive liquid-crystalline materials with small layer shrinkage at the transition to the ferroelectric phase

A homologous series of liquid-crystalline compounds with an azo-group in the molecular core have been synthesized. A tendency for the layer shrinkage at the SmA-SmC* phase transition to decrease with decreasing alkyl chain length (n) was observed, with one homologue exhibiting almost no change in layer spacing. All studied compounds exhibited the photoferroelectric effect, namely a lowering of the spontaneous polarization and tilt angle in the SmC* phase under illumination by visible light. This effect is stronger for homologues with lower n and the time constant of this process, connected with the trans-cis isomerization of the azo group, follows the Arrhenius law. In addition to these effects, a lowering of the phase-transition temperature to the ferroelectric phase by a few Kelvin also occurs. Under sufficiently high light intensity, the ferroelectric SmC* phase completely disappears. After the light is switched off, the spontaneous quantities are restored to their original values.